This invention relates to lithiated metal oxide intercalation compounds, and particularly to lithium manganese oxides with spinel structures for positive electrodes in 4 V secondary lithium and lithium-ion batteries.
Lithium manganese oxide spinel compounds such as Li1+X+Mn2xe2x88x92XO4+y have been used as positive electrode material for 4 V secondary lithium and lithium-ion batteries. Typically, these spinel compounds are formed by firing (calcining) a mixture of a manganese source compound and a lithium source compound. Exemplary manganese source compounds include manganese carbonate (MnCO3), electrochemical manganese dioxide (xcex3-Mn O2 or EMD), and chemical manganese dioxide (xcex3-MnO2 or CMD).
As described in coassigned U.S. Pat. No. 5,789,115, the mean particle size and particle size distribution of these compounds and, in particular, Li1+XMn2xe2x88x92XO4+Y, is dependent on the mean particle size and particle size distribution of the raw materials used to make these compounds and specifically the manganese source compound. In addition to affecting the particle size and particle size distribution of the lithium manganese oxide, the morphology, e.g., density and porosity, of the manganese source compound can affect the morphology of the resulting lithium manganese oxides. In particular, the crystal growth of the spinel phase using a low density manganese compound causes an increase in the distance between the spinel crystallites and has a negative effect on the final density of the spinel compound. This presents a problem with MnCO3 and CMD because these manganese source compounds have relatively low densities and thus produce a low density product. Because EMD has a higher density than MnCO3 and CMD, EMD is often used instead of these manganese source compounds to produce spinel compounds. Nevertheless, the combined water, porosity and vacancies in the EMD structure have a negative effect on the density of the resulting spinel compound.
The morphology of the manganese source compound also affects the tap and pellet density of the spinel compound. The tap and pellet density are important properties characterizing positive electrode materials for secondary lithium and lithium-ion batteries. In particular, these properties directly influence the specific cell energy, cell safety performance, manganese dissolution, capacity fade and capacity loss at room and elevated temperatures, for the electrochemical cell. Therefore, providing a method for preparing lithium manganese oxide spinel compounds having a desired tap and pellet density is of great importance in developing high energy density and high electrochemical performance 4 V secondary lithium and lithium ion batteries.
The present invention is directed to lithium manganese oxide spinel compounds having a low porosity, a high tap density and a high pellet density, and a method of preparing these compounds. In particular, the method comprises preparing a lithium manganese oxide with a spinel structure and having the formula:       Li          1      +      X        ⁢      Mn          2      -      Y        ⁢      M          m      1        1    ⁢      M          m      2        2    ⁢  …  ⁢      xe2x80x83    ⁢      M          m      k        k    ⁢      O          4      +      Z      
wherein:
M1, M2, . . . Mk are cations different than lithium or manganese selected from the group consisting of alkaline earth metals, transition metals, B, Al, Si, Ga and Ge;
X, Y, m1, m2, . . . mk are molar parts, each having a value between 0 and 0.2;
Z is a molar part having a value between xe2x88x920.1 and 0.2; and
the molar parts X, Y, m1, m2. . . mk are selected to satisfy the equation:
xe2x80x83Y=X+m1+m2+. . . +mk 
These lithium manganese oxide compounds are produced by calcining a mixture comprising at least one manganese oxide (manganese source compound) selected from the group consisting of Mn2O3 or Mn3O4, at least one lithium compound, and optionally at least one M1, M2, . . . Mk source compound, in at least one firing step at a temperature between about 400xc2x0 C. and about 900xc2x0 C.
The manganese oxide compounds can be formed by firing highly crystalline xcex2-MnO2 at a temperature between about 500xc2x0 C. and 1000xc2x0 C. Preferably, the xcex2-MnO2 is fired at a temperature between about 600xc2x0 C. and about 800xc2x0 C. in the preparing step to form Mn2O3 manganese oxide. The highly crystalline xcex2-MnO2 used to produce the Mn2O3 or Mn3O4 is preferably formed by firing Mn(NO3)2 at a temperature between about 200xc2x0 C. and about 400xc2x0 C. to thermally decompose the Mn(NO3)2 and form xcex2-MnO2. In addition, the xcex2-MnO2 preferably has a mean particle size of between about 5 xcexcm and about 20 xcexcm and can be ground to produce this mean particle size.
In the calcining step, the mixture of source compounds is fired at between about 400xc2x0 C. and about 900xc2x0 C. Preferably, the mixture is calcined using more than one firing step at firing temperatures within this temperature range. During calcination, agglomeration of the spinel particles is preferably prevented. For example, during a multiple step firing sequence, agglomeration can be prevented by firing the source compounds in a fluid bed furnace or rotary calciner during at least a portion of the firing steps or by grinding the spinel material between steps. The lithium manganese oxide spinel compounds of the invention can be used as positive electrode material for a secondary lithium or lithium-ion electrochemical cell.
The lithium manganese oxide spinel compounds of the invention have a high tap density and pellet density and a low porosity and specific area. In addition, these compounds have a high specific capacity, low capacity fade during cycling, and a low capacity loss during storage at room and elevated temperatures. In particular, the spinel compounds of the invention have a tap density of greater than 1.9 g/cm3 and preferably greater than 2.1 g/cm3. The pellet density for those spinel compounds is greater than 2.85 g/cm3, preferably greater than 2.90 g/cm3, or even greater than 2.95 g/cm3. The pore volume of the pores having a mean radius of less than 1 micron in the spinel compound is no more than 20%, preferably no more than 15% or even no more than 10%, of the total pore volume of the spinel compound, thus illustrating the low porosity of the spinel compound. In addition, the specific area of the spinel compound is less than about 0.8 m2/g and preferably less than 0.6 m2/g or even less than 0.5 m2/g.
The present invention also includes the Mn2O3 and Mn3O4 manganese oxide compounds used to produce the spinel compounds of the invention. These manganese oxide compounds are highly crystalline and have a low porosity. In particular, these manganese oxide compounds have a porosity such that the pore volume of pores having a mean radius of less than 1 micron in said manganese oxide is no more than 20% of the total pore volume of said manganese oxide. These manganese oxides also have a specific area of less than 2.0 m2/g, preferably less than 1.5 m2/g or even less than 1.0 m2/g. The tap density of the manganese oxides is preferably greater than 2.2 g/cm3, more preferably greater than 2.4 g/cm3.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings which describe both the preferred and alternative embodiments of the present invention.